This invention relates generally to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to simultaneously provide stabilization and continued flexibility and proper anatomical motion, and more specifically to such a device which utilizes a spirally slotted and rotatably mounted belleville washer as a restoring force generating element.
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art which achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
Referring now to FIGS. 1 and 2, in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages 10 generally comprise tubular metal body 12 having an external surface threading 14. They are inserted transverse to the axis of the spine 16, into preformed cylindrical holes at the junction of adjacent vertebral bodies (in FIG. 2 the pair of cages 10 are inserted between the fifth lumbar vertebra (L5) and the top of the sacrum (S1). Two cages 10 are generally inserted side by side with the external threading 14 tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (S1) below. The cages 10 include holes 18 through which the adjacent bones are to grow. Additional material, for example autogenous bone graft materials, may be inserted into the hollow interior 20 of the cage 10 to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage 10.
These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient""s spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which nearly completely mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.
It is, therefore, an object of the present invention to provide a new and novel intervertebral spacer that stabilizes the spine without promoting a bone fusion across the intervertebral space.
It is further an object of the present invention to provide an implant device which stabilizes the spine while still permitting normal motion.
It is further can object of the present invention to provide a device for implantation into the intervertebral space that does not promote the abnormal distribution of biomechanical stresses on the patient""s spine.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.
The preceding objects of the invention are achieved by the present invention which is a flexible intervertebral spacer device comprising a pair of spaced apart base plates, arranged in a substantially parallel planar alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) and coupled to one another by means of a spring mechanism. In particular, this spring mechanism provides a strong restoring force when compression and/or lateral deflection loads are applied to the plates, and also permits rotation of the two plates relative to one another. While there are a wide variety of embodiments contemplated, a preferred embodiment includes a belleville washer utilized as the restoring force providing element, the belleville washer being spirally slotted and rotatably mounted.
More particularly, as the assembly is to be positioned between the facing surfaces of adjacent vertebral bodies, the base plates should have substantially flat external surfaces that seat against the opposing bone surfaces. In as much as these bone surfaces are often concave, it is anticipated that the opposing plates may be convex in accordance with the average topology of the spinal anatomy. In addition, the plates are to mate with the bone surfaces in such a way as to not rotate relative thereto. (The plates rotate relative to one another, but not with respect to the bone surfaces to which they are each in contact with.) In order to prevent rotation of a plate relative to its adjacent bone, the upper and lower plates alternatively may each include outwardly directed spikes or ridges that penetrate the bone surface and mechanically hold the plates in place. However, it is more preferably anticipated that the plates should include a porous feature into which the bone of the vertebral body can grow. The most desirable upper and lower plate surface porous feature is a deflectable wire mesh into which the bone can readily grow, and which mesh will deform to seat into the concave upper and lower bone faces. (Note that this limited fusion of the bone to the base plate does not extend across the intervertebral space.) These features, while being preferred, are not required.
In some embodiments (although not in the preferred embodiment), between the base plates, on the exterior of the device, there is included a circumferential wall which is resilient and which simply prevents vessels and tissues from entering within the interior of the device This resilient wall may comprise a porous fabric or a semi-impermeable elastomeric material. Suitable tissue compatible materials meeting the simple mechanical requirements of flexibility and durability are prevalent in a number of medical fields including cardiovascular medicine, wherein such materials are utilized for venous and arterial wall repair, or for use with artificial valve replacements. Alternatively, suitable plastic materials are utilized in the surgical repair of gross damage to muscles and organs. Still further materials that could be utilized herein may be found in the orthopedic field in conjunction with ligament and tendon repair. It is anticipated that future developments in this area will produce materials that are compatible for use with this invention, the breadth of which shall not be limited by the choice of such a material.
As introduced above, the internal structure of the present invention comprises a spring member, which provides a restoring force when compressed or laterally deflected. The restoring force providing subassembly does not substantially interfere with the rotation of the opposing plates relative to one another. In the preferred embodiment, the spring subassembly is configured to allow rotation of the plates relative to one another. As further mentioned above, the force restoring member comprises at least one belleville washer that is spirally slotted.
Belleville washers are washers that are generally bowed in the radial direction. Specifically, they have a radial convexity (i.e., the height of the washer is not linearly related to the radial distance, but may, for example, be parabolic in shape). The restoring force of a belleville washer is proportional to the elastic properties of the material. In addition, the magnitude of the load support and restoring force provided by the belleville washer under compression and/or lateral deflection may be modified by providing one or more slots in the washer. In the preferred embodiment of the present invention, the belleville washer utilized as the load supporting and force restoring member is spirally slotted, with a single spiral slot initiating near the periphery of the washer and extending along an arc that is radially inwardly directed a distance toward the center of the bowed disc, and terminating near the center of the bowed disc. Preferably, the spiral slot extends around the surface of the belleville washer for more than 360 degrees and preferably 540 degrees. Additional configurations, including multiple slots, arcs of different lengths and/or arcs that extend for more or less than 360 degrees, can be used to adjust the load bearing and force restoring characteristics of the belleville washer within the scope of the present invention.
In the preferred embodiment of the present invention, a single belleville washer, which is slotted as described above, is utilized in conjunction with a rotational mounting between one end of the belleville washer and one of the opposing plates, and a rigid fixation of the other end of the belleville washer to the other of the opposing plates. The rotational mounting allows the washer to freely rotate relative to the one of the opposing plates. In as much as the washer is rigidly fixed to the other of the opposing plates, the mounting allows the opposing plates to rotate relative to one another. More particularly, this embodiment comprises a pair of spaced apart base plates, one of which is a disc shaped member (preferably shaped to match the end of an intervertebral disc) having an external face (preferably having the porous coating discussed above) and an internal face. The wide end of the belleville washer is rigidly fixed to the internal face of this base plate, preferably by laser welding. The other of the base plates is similarly shaped, having an exterior face (preferably having the porous coating discussed above), but further includes on its internal face a central post which rises out of the internal face at a nearly perpendicular angle (it should be understood that the post need not extend from the center of the plate, but rather is can be positioned according to the proper clinical placement depending on where the device is placed in the spine, In as much as a more anterior or a more posterior position may be suitable in certain parts of the spine). The central post comprises a plurality of upwardly extending members that mutually define a cylinder having a central axial bore and vertically oriented slots separating each individual member. This conformation permits the mutually defined cylindrical shape to deflect inward upon the application of a corresponding force and return to an undeflected shape once the force is relieved. Each of the upwardly extending members comprises a generally uniform radial thickness, thereby mutually defining a constant diameter for the cylinder from its union with the plate up to a circumferential position near to the uppermost extent thereof. The uppermost extent thereof, however, comprises a discontinuously widened circumference that subsequently tapers radially inwardly from that vertical position to the upper end. This discontinuously widened circumference thereby defines an annular ledge around the cylindrical top section, which ledge tapers inwardly to provide a beveled conformation. The portion of the post from the ledge to the upper end of the post is referred to herein as the head of the post. The central axial bore is threaded, and receives a small set screw. Prior to the insertion of the set screw, the post can deflect radially inward because of the axial bore and the vertically oriented slots. The insertion of the set screw eliminates the capacity for this deflection
As introduced above, the spirally slotted belleville washer is mounted to this central post in such a way that it may rotate freely through a range of rotation angles equivalent to the fraction of normal human spine rotation (to mimic normal disc rotation). In this regard, the belleville washer has a flattened narrow end with a central opening. The central opening has a diameter that is greater than the diameter of the post up to the ledge, but smaller than the diameter of the head at the ledge. Therefore, the head can be passed through the central opening when the set screw is not in the axial bore, because the slots will allow the head to deflect inward when the head is forced through the central opening. Once the head has passed through the central opening, the head will return to its undeflected shape so that the narrow end is seated between the plate and the ledge. Subsequent introduction of the set screw into the axial bore eliminates the capacity for the head to deflect. Preferably, the length of the post from the plate to the ledge is slightly longer than the thickness of the washer at the narrow end, so that the washer is restricted from angulating with respect to the plate but not restricted from rotating with respect to the plate. (Angulation of the plates relative to one another will be possible because of the ability of the washer to deflect under lateral deflection forces and return to its undeflected shape.)
The assembly provides ample spring-like performance with respect to compression and lateral deflection loads, as well as long cycle life to mimic the biomechanical performance of the normal human intervertebral disc. The rigid fixation of the wide end of the belleville washer maintains the wide end against the one opposing plate. While the narrow end of the belleville washer can rotate freely relative to the other opposing plate, the narrow end is angulationally fixed relative to that plate (as described above). Therefore, not only compression, but also lateral deflection loads, are borne by the washer spring. The spiral slot of the belleville washer allows the washer to compress as the slot narrows under compression loads, only to spring back into its undeflected shape upon the unloading of the spring. Further, the spiral slot allows one side of the washer to compress and the opposite side to expand as the portion of the slot on the one side narrows and the portion of the slot on the opposite side widens under lateral deflection loads, only to spring back into its undeflected shape upon the unloading of the spring.
Finally, In as much as the human body has a tendency to produce fibrous tissues in perceived voids, such as may be found within the interior of the present invention, and such fibrous tissues may interfere with the stable and/or predicted functioning of the device, some embodiments of the present invention (although not the preferred embodiment) will be filled with a highly resilient elastomeric material. The material itself should be highly biologically inert, and should not substantially interfere with the restoring forces provided by the spring-like mechanisms therein. Suitable materials may include hydrophilic monomers such as are used in contact lenses. Alternative materials include silicone jellies and collagens such as have been used in cosmetic applications. As with the exterior circumferential wall, which was described above as having a variety of suitable alternative materials, it is anticipated that future research will produce alternatives to the materials described herein, and that the future existence of such materials which may be used in conjunction with the present invention shall not limit the breadth thereof.